F7P (1.8 16v) are cheap as chips and always available. Try and get a goodun and many of them will benifit from a top end rebuild.
F7R (2.0 16v) - same engine, longer stroke, noticably more usable power, more expensive, williams or megane - megane runs distributorless though so there are management issues.
F4R - 1*2 engine - Bit more of a challenge - you need the cable throttle body, you need to find a place to hack the immo off the ECU. All the mounts line up but the wiring can be a b*****d. F0xy is probably the best person to speak to about wiring.
C1J - R5 turbo - I wouldn't. Boost+carb = difficulties. has been done tho
Then theres the diesels that make for a very interesting project